A chemical looping combustion process generally comprises one or more reaction zones wherein combustion of a fuel is conducted by contacting with an oxygen-carrying solid that is thereafter re-oxidized in at least one oxidation zone by contacting with air or water vapour prior to being sent back to the combustion (or reduction) zone(s).
Chemical looping combustion thus consists in contacting in an enclosure at high temperature a gaseous, liquid and/or solid hydrocarbon feed with an oxygen-carrying metallic oxide type solid. The metallic oxide yields part of the oxygen it contains, which takes part in the combustion of the hydrocarbons.
After combustion, the fumes contain predominantly carbon oxides, water and possibly hydrogen. In fact, it is not necessary to contact the hydrocarbon feed with air and the fumes are then predominantly made up of combustion gases and possibly of a dilution gas used for transport and fluidization of the particles (water vapour for example).
It is thus possible to produce predominantly nitrogen-free fumes with high CO2 contents (>90 vol. %) allowing to consider CO2 capture, then storage. The metallic oxide that has taken part in the combustion is then carried to another reaction enclosure where it is contacted with air in order to be re-oxidized.
Implementing a chemical looping combustion method requires large amounts of metallic oxides in order to burn all of the fuel. These metallic oxides are generally contained either in ore particles or in particles resulting from industrial treatments (residues from the iron and steel industry or from the mining industry, used catalysts from the chemical industry or refining). It is also possible to use synthetic materials such as, for example, alumina or silica-alumina supports on which metals that can be oxidized (nickel oxide for example) have been deposited.
The maximum oxygen capacity really available varies considerably from one oxide to another, generally ranging between 0.1 and 15%, often between 0.3 and 6 wt. %. Implementation in fluidized bed mode is therefore particularly advantageous for conducting the combustion. In fact, the finely divided oxide particles circulate more readily in the combustion and oxidation reaction enclosures, and between these enclosures, if the properties of a fluid are conferred on the particles.
Patent application FR-2,850,156 describes a chemical looping combustion method wherein the fuel is crushed prior to being fed into the reduction reactor operating in circulating fluidized bed mode. The reduced size of the solid fuel particles allows more complete and faster combustion, and it allows to produce nearly 100% fly ashes that are separated from the circulating oxides. Separation downstream from the circulating bed is first provided by a cyclone, then by a device allowing the unburnt particles to be separated from the metallic oxide particles. Unburnt particles entrainment in the oxidation zone and therefore CO2 emissions in the oxidation reactor effluents is thus prevented.
The separation device comprises a bed fluidized by water vapour, which allows to separate the fine and light particles such as carbon-containing residues and to feed them into the reactor again, whereas the denser and bigger oxide particles are transferred to the oxidation reactor. This device is a relatively sophisticated equipment since it contains two internal compartments.
Furthermore, according to document FR-2,850,156, the fly ashes are separated from the oxide particles in a second circuit where a separator operating in fluidized bed mode conducts the separation, the fluidized fly ashes being sent to a silo via a pneumatic transport device and the metallic oxides being extracted at the base of the fluidized-bed reactor after decanting.
Besides, the high gas velocities involved in the reduction reactor operating in circulating fluidized bed mode do not allow to obtain sufficient particle residence times for gasification of all the solid fuel, then for combustion of the gasification products. Significant recycling of the unburnt particles by passage through the separator is therefore necessary. Now, separating the unburnt particles from the oxide particles is a delicate operation because it requires supplying additional gas in large amounts, which is energy-consuming.
Furthermore, due to the too short residence time, it is difficult to achieve total combustion and the fumes contain large amounts of CO and of H2, which involves the presence of a post-combustion zone downstream from the process.
N. Berguerand's thesis “Design and Operation of a 10 kWth Chemical-Looping Combustor for Solid Fuels”, ISBN 978-91-7385-329-3, describes a device allowing to carry out coal combustion using a chemical loop.
This device is made up of an oxidation reactor using metallic particles, a cyclone allowing separation of the particles and of the depleted air after oxidation, a fluidized bed supplied with oxidized metallic oxides through the return leg arranged below the cyclone, wherein reduction of the metallic oxide is carried out through combustion of the coal. The coal is fed to the upper part of the fluidized bed, in the dilute phase. In the reduction reactor, combustion of the coal occurs progressively: the coal particles start flowing down and are devolatilized in the dilute phase, counter-current to the fluidization gases, wherein the metallic oxides are present only in small amounts ; they are then contacted with the fluidized metallic oxides in the dense phase. The long residence time allows to gasify the coal and to produce combustion gases containing large amounts of carbon monoxide and of hydrogen that pass into the dilute phase.
In the dense phase of the reactor, the fluidization rates are low—generally ranging between 5 and 30 cm/s—, which does not allow the entrainment of significant amounts of metallic oxides in the dilute phase likely to promote the combustion of gases such as CO, H2 or the volatilized hydrocarbons that are discharged from the dilute zone. The amounts of CO and of hydrocarbons (HC) in the reduction reactor effluents are therefore significant and above several percents by volume. The combustion efficiency is thus not very good and a post-combustion zone is also necessary to complete the combustion.
Furthermore, according to this document, the reduction reactor is equipped with a particle separator integrated in the dense phase, which requires supplying additional gas for separation.
The applicants have developed an improved chemical looping combustion method that allows, even from fuel particles in coarse state, to obtain total combustion of the solid feed while minimizing the amount of solid feed to be recycled, which allows the energy efficiency of the method to be maximized.
The combustion method according to the invention allows to capture at least 90% of the CO2 emitted by the combustion in the fumes directly at the combustion reactor outlet, the capture rate being defined by the ratio:
                    ⁢                                        Amount            ⁢                                                  ⁢            of            ⁢                                                  ⁢                          CO              2                        ⁢                                                  ⁢            emitted            ⁢                                                  ⁢            in            ⁢                                                  ⁢            the            ⁢                                                  ⁢            fumes                                                            from            ⁢                                                  ⁢            the            ⁢                                                  ⁢            combustion            ⁢                                                  ⁢            reactor                                                                        Amount            ⁢                                                  ⁢            of            ⁢                                                  ⁢                          CO              2                        ⁢                                                  ⁢            emitted            ⁢                                                  ⁢            in            ⁢                                                  ⁢            the            ⁢                                                  ⁢            chemical                    ⁢                                                                              looping          ⁢                                          ⁢          combustion          ⁢                                          ⁢          process                    
At the outlet of the combustion process according to the invention, the CO/CO2 molar ratio of the fumes downstream from the cyclones is below 0.05 and the H2/H2O ratio is below 0.05.
In the chemical looping combustion method according to the invention, on the one hand, contact between the particles carrying the oxygen and the solid fuel is optimized to promote the coal gasification reactions and, on the other hand, contact between the gasification products and the metallic oxides is also optimized so as to produce effluents that have undergone total combustion (H2, CO and HC<1 vol. % in the fumes).
Besides, according to the invention, separation of the unburnt particles from the metallic oxide particles is carried out upstream from the reduction reactor fumes dedusting stage so as to make best use of the maximum kinetic energy of the fumes for separation of the two types of particle.